Ideally, you should have a broadband connection ( > 128 kpbs). There are various reasons for this:
1. You need at least 64 kbps download plus 64 kbps upload bandwidth to make an uncompressed VoIP call. Otherwise you will need to compress your VoIP traffic using compression codecs such as G.723 or G.729 which can decrease the bandwidth to approximately 20 kbps. However, such methods of compression are usually used in situations where multiple calls are going over the same connection, not when bandwidth is restricted to under 128 kbps. If you have dial-up you will be very fortunate to achieve decent call quality, even when using compression.
2. A broadband connection is usually 'always-on' thereby ensuring that you can receive and make calls with minimum interruption.
3. Depending no how you use your Internet connection, you want to make sure that you have enough bandwidth leftover for typical Internet traffic (eg emails, web browsing, file downloads)
If you are unsure of how your Internet connection will be able to cope, try testing your connection for VoIP here.